The structure and behavior of the energetic electrons and protons trapped in Earth's Radiation Belt (RB) has been the subject of numerous experimental and theoretical studies. Morphologically, two regions are distinguished in an ionosphere such as an inner RB for L shells lower than two and an outer RB for L shells higher than two. The inner RB is dominated by protons with energy in excess of 10 MeV and lifetimes from a few years at low altitudes of 400 to 500 km to many tens of years at higher altitudes. Overall the inner belt energetic protons are relatively stable with a typical lifetime of ten years. Contrary to this, the outer RB is very dynamic and dominated by energetic electron fluxes associated with solar events and space weather process.
Earth's inner radiation belt located inside L=2 is dominated by a relatively stable flux of trapped protons with energy from a few to over 100 MeV. Radiation effects in spacecraft electronics caused by the inner radiation belt protons are the major cause of performance anomalies and lifetime of Low Earth Orbit satellites. For electronic components with large feature size, of the order of a micron, anomalies occur mainly when crossing the South Atlantic Anomaly (SAA). However, current and future commercial electronic systems are incorporating components with submicron size features. Such systems cannot function in the presence of the trapped 30 to 100 MeV protons, as hardening against such high-energy protons is essentially impractical.
Low Earth Orbiting (LEO) satellites spend a significant part of their orbit in the inner RB that is populated by energetic protons with energy, from one to more than one hundred MeV. The interaction of energetic protons with electronic devices of modern spacecraft results in high rates of anomalies due to Single Event Effects (SEE). Such anomalies range from nuisance effects that require operator intervention to debilitating effects leading to functional or total loss of the spacecraft. A set of operational problems occur when protons deposit enough charge in a small volume of silicon to change the state of memory cell, so that a one becomes zero and vice versa. The memories can become corrupted and lead to erroneous commands. Such soft errors are referred to as Single Event Upsets (SEU) and often generate high background counts to render the sensor unusable. Sometimes a single proton can upset more than one bit giving rise to Multiple Bit Upsets (MBU). Some devices can be triggered into a high current drain, leading to burn-out and hardware failure, known as single event latch-up or burn-out. Other devices suffer dielectric breakdown and rupture.
For LEO satellites, the dominant source of proton influence is the South Atlantic Anomaly (SAA). The SAA is a localized region at a fixed altitude, where protons in the inner RB reach their maximum intensity as a result of the asymmetry of the earth's magnetic field that can be approximated by a tilted, offset dipole in the inner magnetosphere. At present, satellites with micron size Commercial-Off-The Shelf (COTS) electronics experience serious effects mainly when transiting the SAA. For example, the intolerable frequency of SEU of the IBM 603 microprocessors (5 micron CMOS) in Iridium forced Motorola to disable the cache while transiting the SAA. Similar anomalies were experienced by the Hubble Space Telescope and numerous other satellites. To mitigate such effects, spacecraft utilize shielded electronic components that can reduce the flux of protons with energy below few MeV. However, it is very hard to shield against proton fluxes with energy in excess of 20 to 30 MeV.
The severity of the environment is usually expressed as an integral linear energy transfer spectrum, that represents the flux of particles depositing more than a certain amount of energy and charge per unit length of the material. This is referred as Linear Energy Transfer (LET), and given in units of MeV per g/cm2 or per mg/cm2. The effect on devices is characterized as a cross section (effective area presented to a beam), that is a function of the LET. The frequency of SEU caused by energetic protons is a non-linear function of the feature size. For large feature sizes, SEU are due to charge deposition caused by secondary particles with higher LET. For feature sizes smaller than 90 nm, direct proton ionization can cause SEU, resulting in an increase of the frequency of proton SEU by two or more orders of magnitude for deep submicron devices. This could preclude their use even for orbit latitudes different than the SAA. Further hardening the microelectronic components, besides the added weight, is very ineffective for proton energies higher than 20 to 30 MeV. For example, even one inch of Al reduces the 60-80 MeV flux by less than a factor of three. The recent tests have shown that the SEU cross section for energies between 1 to 10 MeV for bulk 65 nm Complementary Metal Oxide Semiconductor (CMOS) technology is by two orders of magnitude higher than for micron size devices, rendering current shielding level inadequate even at low proton energies.
Use of COTS in space applications is dictated by their high volume production and wide-spread use. The high volume production drives down their recurring component costs because of high yields and economies of scale. The wide-spread use of COTS reduces the system cost. Furthermore open standards drive down development and life-time support costs reduce the time to market for new products. The SEE issue for submicron CMOS or other electronic components presents a major dilemma, since it will prohibit use of COTS circuits with sub-micron size features and will limit the use of micro-satellites at LEO orbits.
Thus it is difficult to shield against 30 to 100 Mev protons to the level required by sub-micron features of current and future commercial electronic components. Heavy weight penalty must be paid to effect such shielding. Therefore, it is believed that a need exists for an improved system and method for reducing the energetic proton flux trapped in the inner radiation belt. Such system and method should allow the use of commercial electronics with submicron feature size on Low Earth Orbit (LEO) satellites and microsatellites without the operational constraints imposed by the presence of energetic proton fluxes trapped at the inner radiation belts.